The present invention relates to a field of the art dealing with vacuum ultraviolet (VUV) line radiation. Representative examples of the art in this area appear in "Techniques of Vacuum Ultraviolet Spectroscopy" by J. R. Sampson & Wiley 1967-New York.
Vacuum ultraviolet line radiation has been produced by a wide variety of techniques. Most techniques are based upon the atomic/ionic excitation and subsequent decay by photo emission that occurs in electrical discharges. Generally, these discharges fall into four broad categories: (a) glows; (b) arcs; (c) R. F./microwave discharges; and (d) high voltage sparks.
(a) Glow Discharges: The most commonly used device, the glow discharge VUV source consists of an insulating refractory tube with electrodes fastened at each of its ends. If a gas or vapor is admitted into the tube at a pressure of a few hundred millitorr and if a potential of a few hundred volts is applied across the electrodes, the gas begins to glow. Such glow discharge sources are usually simple, stable, moderately powered, and can be made to operate with a wide variety of different species. However, these sources are often limited by their low brightness and high operating gas pressures. Therefore, this is not useful for applications that involve grating spectroscopes, windowless multipliers and other devices requiring high vacuums.
(b) Arcs: If the potential across a glow discharge tube is increased, ultimately a further breakdown occurs and the discharge becomes an arc. A magnetic field can be used to axially concentrate the arc, as in the duoplasmatron source, which increases the overall brightness as well as the ionic characteristic radiation yield. The increased brightness of arc sources allows the use of differential pumping to relieve the problems created by the source gas load. However, because of the increased complexity brought about by the addition of magnet and filament systems arc sources have proven too costly for many uses.
(c) R. F.--Microwave Discharges: Energy pumped into a gas in the form of microwaves or R. F. can induce a discharge at power levels comparable to glows and arcs. However, electronic complexity and high operating gas pressures/loads are inherent in the design of R. F./Microwave sources. Other fundamental limitations on available brightness due to gas-energy coupling considerations are also present in such devices. This combination of difficulties in applications of R. F. sources to cases in which their high spectral purity is a dominant consideration.
(d) High Voltage Sparks: At reduced pressures the application of a high potential (several kilovolts) across a gas column results in a spark discharge. The combination of high potential and long mean free path allows the electrons and ions in the spark to attain energies sufficient for multiple collisional ionization. Hence, the resulting VUV spectro are rich in lines characteristic of highly stripped species. Spark sources have been manufactured for a wide variety of different species. Nearly all involve very high temperatures, high power levels and rapid erosion of source components. These difficulties mandate use of refractory construction, extensive cooling, and pulsed operation. In most applications the gas load imposed by these sources is seldom a problem.
Accordingly, the present invention may be viewed as a means for providing an improved vacuum ultraviolet lamp system that would enable the prior art to overcome the basic shortcomings as have been enumerated above.